If we cannot get people to Mars, the next best option is to bring Mars to us. A few rocks delivered to labs on Earth could lead to a drastically deeper understanding of the Red Planet's geology, its potential to support life, and its suitability as a target for future astronauts. "I'm convinced that a robotic sample return is the single biggest step that we as planetary scientists could take to foster human exploration of Mars," says planetary scientist and former NASA associate administrator Alan Stern.

The National Research Council agrees. In March the nonprofit advisory group recommended making sample return one of NASA's top priorities for planetary science. NASA plans to hone its sample-return skills in 2016 with OSIRIS-REx, a boxy spacecraft that will land on an asteroid, collect chunks of rock and dust, and then return to Earth.

By 2018 NASA and the European Space Agency hope to launch the first phase of bringing back Mars samples. Cost concerns have kept the details of the mission in flux, but at its most basic, it will probably entail a rover to collect samples, an ascent vehicle to carry the payload into orbit around Mars, and a spacecraft to carry the precious cargo home.